1. Field of the Invention
The present invention relates to a torque sensor of the type which is arranged to detect torsion by detecting the relative displacement of a first shaft and a second shaft which are connected via a connecting means.
2. Description of the Related Art
Proposals have heretofore been made with respect to an apparatus of the type which is arranged to detect the direction of and the amount of variation in the relative angular variation between first and second shafts which are connected via a torsion bar serving as connecting means and then to impart to the second shaft a driving force which corresponds to the result of that detection.
For example, a known type of torque sensor for power-steering purposes is disclosed in U.S. Pat. No. 4,448,275 entitled "TORQUE SENSOR". This torque sensor is constructed using a sliding contact for detecting a variation in the relative angle between a first shaft--an input shaft through which a rotational force is transmitted from a steering wheel and a second shaft--an output shaft which is connected to a steering gear.
The torque sensor of this slide contact type comprises a detecting portion consisting of a slide contact, a resistive wire and the like, all of which cooperate to detect a variation in the relative angle between the input shaft and the output shaft and a pickup portion consisting of a slip ring, a brush and the like which cooperate to transmit to the outside the detection result of the variation in the relative angle between the first shaft and the second shaft.
In practical examples based on this proposal, the detecting portion and the pick portion, as the basic principle, rely on the above-described slide-contact system. Accordingly, to meet safety standards for vehicles, the slip ring is plated with gold and the resistive wire is formed on a ceramic board which is expensive but excellent in temperature characteristics.
However, the torque sensors of the slide-contact type are expensive and, in addition, are of a contact type. Accordingly, when compared with a non contact type of torque sensor, the problem of abrasion of a brush or a resistor is inevitably encountered, and noise or the like tends to easily occur due to dust produced as the result of the abrasion. It follow, therefore, that the slide-contact type of torque sensor is inferior in reliability or durability. Moreover, since the brush which constitutes part of a detector rotate together with the shafts, in addition, since curled cords are employed as lead wires for outputting purposes, the service life of such a torque sensors has been limited due to the necessity for repetition of winding operations.
In contrast, U.S. Pat. No. 4,784,002 entitled "TORQUE SENSOR" discloses a torque sensor of the non-contact type in which a magnetism sensor is employed in place of the slide contact described above The torque sensor disclosed in this prior patent is arranged including "a first shaft having a fore end portion which is constructed such that a displacement of torsion thereof occurs; a second shaft connected to the first shaft; a predetermined number of N poles and S poles arranged as fixed magnetic poles in alternating sequence so that they surround the periphery of this connecting part; first magnetism pickup paths and second magnetism pickup paths corresponding in number to the N poles and the S poles, respectively, and being located in opposing relationship midway between adjacent poles; and a plurality of magnetism detecting elements provided in non-contact relationship to the first shaft for detecting a change in magnetic flux passing through the first and second pickup paths, at least two of the aforesaid magnetism detecting elements being provided at predetermined angular intervals about the axis of the first shaft so as to correct mechanical decentering with respect to that axis on the first and second pickup paths. This conventional type of torque sensor is arranged so that, when the N poles approach either the first pickup paths or the second pickup paths due to the torsional displacement of the second shaft, the quantity of magnetic flux passing through the first and second pickup paths is varied and the torsional displacement of the first shaft with respect to the second shaft is thereby detected from this variation in the magnetic flux".
The torque sensor employing the above-described magnetism sensor, however, involves a number of problems; firstly the magnetism detecting elements must be provided at predetermined angular intervals about the axes of the first and second shafts; and secondly the N and S poles, each consisting of a single magnet, must be alternately disposed as fixed magnetic poles. In addition, the ease of assembly is remarkably restricted owing to the first and second problems and the number of parts inevitably increases. As a result, the manufacturing cost will increase.